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The input/output capacitor calcaulating formula of LDO(LM22672)

user4195297
Prodigy 60 points
Other Parts Discussed in Thread: LM22672, LM22670, LM46002

Dear engineer:

Hello! I am a hardware engineer of Emerson, we selected LM22672 as our power chip, but I don't know how to choose input and output capacitor of LM22672, such as type, value of capacitors and so on, could you supply detail information for me, any information will be appreciated, thanks.

Mervyn

  • 0
    TI__Genius 16025 points

    Hi Mervyn,

    Please go the LM22672 webpage and use WEBENCH to calculate your circuit requirements. It will simulate it for you and provide layouts and schematics.

    It will look something like this:

    Thanks,

    Anston

  • 0 in reply to Anston Lobo
    Prodigy 60 points

    Hi Anson:

    At first, thanks for your replay, I can get circuit from input parameters to WEBENCH. But I still have a question, how WEBENCH caculate parameters. Can you supply some details, thank you.

    Mervyn

  • 0 in reply to user4195297
    TI__Genius 16025 points

    Mervyn,

    We put the Pspice model in this tool. It also has a huge passives library that allows us to select the best component available.

    If something is unclear, please specify exactly what you need.

    Thanks,

    Anston

  • 0 in reply to user4195297
    TI__Genius 15810 points

    Hello,

    The input capacitor for a buck regulator (e.g.LM22670) is selected such that it's capacitance does not allow for a large deviation on the input voltage, it's RMS current rating is capable of handling the worst case input RMS current and it's voltage rating should be well above the max input voltage.

    CinMin = Iout*Ton/deltaVin
    IrmsMin = 1.25*{Iout*sqrt(D*(1-D))}
    Vdc = 1.25*VinMax
    ESRmax = 

    The output capacitor is part of the output filter and also provides the charge storage for the load. Calculation of Cout depends heavily on the type of architecture, control scheme and whether the device is internally or externally compensated. The following set of equations are generally used to define a minimum capacitance. This just gives us a starting point.

    CoutMin = Iout *deltaT/deltaVout; deltaT is the switching period and deltaVout is the max allowable undershoot. 
    IrmsMin = Iinductorpeaktopeak/sqrt(12)
    Vdc = 1.25*Vout
    ESRMax = Vout ripple/Iinductorpeaktopeak

    This set of equations are very simplistic. I have attached a more comprehensive presentation that talks about selection of output capacitors.

    I hope this information is helpful.

    Regards,
    Akshay

    Capacitors Selection for DC-DC Converters.pptm
  • 0 in reply to Akshay Mehta
    Prodigy 60 points

    Hi Akshay:

    Thank you very much for your detailed reply, the attachment is very helpful to me. But because of my poor knowledge, it is very difficulty to understand the formula. Could you give me some examples about caculating the value and ESR of capaitor, I will be apperciated.

    Best regards.

    Mervyn

  • 0 in reply to user4195297
    TI__Genius 15810 points

    Hello,

    The ESR of the cap matters depending where you use it and it's chemistry. If the cap is used at the output of a voltage regulator, then it's value would affect the stability and the output ripple. If it is used at the input of the voltage regulator, then it's value would affect the power dissipated in the capacitor. 

    For the output capacitor, I wrote the equation in the previous post. Here it is again:
    ESR_Max = Vout_ripple / Inductor_peaktopeak_current
    The above equation defines steady state. The ESR value in an output capacitor would also affect the stability of the design. Ideally, you would want this value to be very small.

    For the input capacitor, the equation is as follows:

    ESR_Max = Iout*Vout*x / Cin_RMScurrent,
    Where, 
    x = Factor of output power which should be very small, e.g. 0.1%. This would ensure that you find an input cap with low enough ESR that will not burn any power.

    For examples on the capacitor calculations, you could refer to the LM46002 datasheet. If your input cap is a part of the input filter, then I'd suggest you to go through the following app note: http://www.ti.com/lit/an/snva489c/snva489c.pdf

    In short, there is no extremely simple explanation for the ESR calculation. The easiest approach is to use MLCC devices as far as possible or use electrolytics whose ESR is lower than the above calculated equations.

    Regards,
    Akshay

  • 0 in reply to Akshay Mehta
    Prodigy 60 points

    Hello Akshay:

    Thank you very much, your patiently answers made me clearly understand. I have a last question to consult you. We adopted LM22672 and I calculated the output capacitance acc. LM22762 datasheet, see the attachment3036.LM22762 output capacitor calculation.docx,

    Please help me to check it, feel free to ask me question, thanks.

  • 0 in reply to user4195297
    TI__Genius 15810 points

    Hello,

    What are your design inputs? From the equation you need a cap larger than 27uF. You'd now need to pick a cap and check the stability with that cap and see if you need to adjust it. Here is where it helps to use the suggestions from Webench. The LM22672 is a voltage model control device which will have a double pole caused by the Cout and L. You could iterate through the capacitance on your lab bench to see what value above 27uF gives you the best performance. But Webench does that already. 

    Regards,
    Akshay

  • 0 in reply to Akshay Mehta
    Prodigy 60 points

    Hi Akshay:

    I put the input parameters and simulation result in the attachment, 4401.Input and simulation.rar

    and I got the output cap is 22uF, why you get the result is 27uF. I think the simulation result is same with my calculation result, am I right. Thanks.

  • 0 in reply to user4195297
    TI__Genius 15810 points

    27uF is a typo. You should be right.

    Regards,
    Akshay

  • 0 in reply to Akshay Mehta
    Prodigy 60 points
    Hi Akshay:

    Thank you very much for your patient and detailed answers, I almost got the answers. You are such a great guy, best wishes for you, thanks.
  • 0 in reply to user4195297
    TI__Genius 15810 points
    Best wishes to you too! Thanks.